Leashes are a safety device only. They keep your dog from running into the street or darting over to a dog or human. The most important thing about a leash is to NOT use it to control your dog--pulling them to you or away from something, repositioning them, or yanking them so they pay attention to you. You may inadvertently teach your dog that the leash is what's important, to only respond when they feel the weight of the leash (after all, the leash is the control device), or worse to resent a leash.
Flat leashes are best. Retractable/flexi leads are designed to allow pulling and distance because there is always a certain amount of tension on the cord and the dog gets used to pulling. When holding a leash, whether standing, sitting or walking, you are a tree with a leash attached to you--your dog cannot move you, or your arm, even an inch. In other words, pulling gets them nothing. Hold the leash in one spot, holding it at your belly button helps remind you not to use it. If you want your dog to move use sound to get their attention (kissy noises, etc.), and if that fails then you move. Show your dog that they need to stay close to you so they know where they're going. Make it all about you and not the leash.
If you've got a dog that bites or chews on the leash think about using a light weight chain leash. They may still chew it but it won't be as fun and you won't keep replacing leashes.
Remember, it needs to be about you and not the leash.